From ad04c03e14df683a8864dfe9ebd4f43cbf8e5891 Mon Sep 17 00:00:00 2001
From: miteruzo <miteruzo@naver.com>
Date: Tue, 3 Nov 2026 13:37:09 +0900
Subject: [PATCH] #414

---
 backend/spec/db/tag_group_constraints_spec.rb |  1 +
 backend/spec/db/tag_version_names_spec.rb     |  1 +
 backend/spec/models/history_access_spec.rb    | 28 +++++--
 backend/spec/models/locale_activation_spec.rb | 25 +++++-
 backend/spec/models/post_version_spec.rb      | 28 +++----
 .../models/tag_name_sanitisation_rule_spec.rb | 27 ++++++
 backend/spec/models/tag_spec.rb               | 28 +++----
 backend/spec/models/version_record_spec.rb    | 46 +++++-----
 backend/spec/rails_helper.rb                  |  1 +
 backend/spec/requests/counts_download_spec.rb |  4 +-
 .../counts_history_boundaries_spec.rb         |  4 +-
 backend/spec/requests/counts_spec.rb          |  8 +-
 backend/spec/requests/counts_table_spec.rb    | 36 ++++++--
 backend/spec/requests/materials_spec.rb       | 41 ++++++++-
 backend/spec/requests/nico_tags_spec.rb       |  5 ++
 backend/spec/requests/posts_spec.rb           | 55 ++++++++----
 backend/spec/requests/sitemap_entries_spec.rb |  4 +-
 backend/spec/requests/tag_children_spec.rb    | 10 +++
 .../requests/tag_description_wiki_spec.rb     |  2 +
 .../spec/requests/tag_descriptions_spec.rb    | 14 +++-
 backend/spec/requests/tag_groups_spec.rb      |  6 +-
 backend/spec/requests/tag_versions_spec.rb    |  1 +
 backend/spec/requests/tags_spec.rb            | 10 +++
 backend/spec/services/counts_export_spec.rb   |  4 +-
 .../services/counts_pivot_security_spec.rb    | 28 +++++--
 .../services/material_sync_importer_spec.rb   |  2 +
 .../services/material_sync_runner_spec.rb     | 41 +++++++++
 .../spec/services/post_bulk_creator_spec.rb   | 83 +++++++++++++++++++
 ...shared_resource_id_allocator_mysql_spec.rb | 10 +++
 .../shared_resource_id_allocator_spec.rb      |  3 +
 backend/spec/services/youtube/sync_spec.rb    | 35 ++++++++
 backend/spec/support/event_records.rb         | 15 ++++
 backend/spec/support/tag_group_records.rb     |  5 +-
 backend/spec/tasks/nico_sync_spec.rb          | 57 +++++++++----
 34 files changed, 554 insertions(+), 114 deletions(-)
 create mode 100644 backend/spec/support/event_records.rb

diff --git a/backend/spec/db/tag_group_constraints_spec.rb b/backend/spec/db/tag_group_constraints_spec.rb
index bdbe417..08472b5 100644
--- a/backend/spec/db/tag_group_constraints_spec.rb
+++ b/backend/spec/db/tag_group_constraints_spec.rb
@@ -102,6 +102,7 @@ RSpec.describe 'Tag group persistence constraints' do
     6.times { nodes = [{ 'tag_id' => tag.id, 'children' => nodes }] }
     reject_statement do
       insert_group_row('tag_group_versions', {
+          event_id: create_event!(event_type: 'unknown').id,
           tag_group_id: group_id, version_no: 2, event_type: 'update', name: '制約確認',
           members: [{ 'tag_id' => tag.id, 'version_no' => tag.version_no }].to_json,
           placements: nodes.to_json, created_at: Time.current })
diff --git a/backend/spec/db/tag_version_names_spec.rb b/backend/spec/db/tag_version_names_spec.rb
index 3c351b4..b90bd84 100644
--- a/backend/spec/db/tag_version_names_spec.rb
+++ b/backend/spec/db/tag_version_names_spec.rb
@@ -10,6 +10,7 @@ RSpec.describe 'TagVersion names database constraint' do
 
   def insert_snapshot!(snapshot)
     TagVersion.insert_all!([{ tag_id: tag.id, version_no: 1, event_type: 'create',
+                             event_id: create_event!(event_type: 'unknown').id,
                              name: 'snapshot_name', category: 'general',
                              aliases: '', parent_tag_ids: '', names: snapshot,
                              created_at: Time.current }])
diff --git a/backend/spec/models/history_access_spec.rb b/backend/spec/models/history_access_spec.rb
index c087c35..9c382fa 100644
--- a/backend/spec/models/history_access_spec.rb
+++ b/backend/spec/models/history_access_spec.rb
@@ -9,7 +9,9 @@ RSpec.describe HistoryAccess do
     locale
     rows = ['ja', 'ain'].flat_map do |language|
       ['create', 'discard', 'restore'].each_with_index.map do |event, index|
-        TagDescriptionVersion.create!(tag_id: tag.id, language_code: language,
+        TagDescriptionVersion.create!(
+            event_id: create_event!(event_type: 'unknown').id,
+            tag_id: tag.id, language_code: language,
             version_no: index + 1, event_type: language == 'ain' ? 'update' : event,
             body: "#{ language } #{ index }", created_at: Time.utc(2024))
       end
@@ -28,7 +30,9 @@ RSpec.describe HistoryAccess do
     public_metadata = described_class.set(user: nil) { described_class.authority }
     admin = create(:user, :admin)
     admin_metadata = described_class.set(user: admin) { described_class.authority }
-    TagVersion.create!(tag:, version_no: 1, event_type: 'discard', name: 'private',
+    TagVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        tag:, version_no: 1, event_type: 'discard', name: 'private',
         category: 'general',
         names: { ja: { name: 'private', script_code: 'Jpan',
                        auto_generated: false, aliases: [] } },
@@ -52,7 +56,9 @@ RSpec.describe HistoryAccess do
     blob = ActiveStorage::Blob.create_before_direct_upload!(filename: 'private.png',
         byte_size: 1, checksum: Base64.strict_encode64('checksum'), content_type: 'image/png')
     ActiveStorage::Attachment.create!(name: 'thumbnail', record: post, blob:)
-    PostVersion.create!(post:, version_no: 1, event_type: 'discard', url: post.url,
+    PostVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        post:, version_no: 1, event_type: 'discard', url: post.url,
         title: 'private', tags: '', tags_json: [], parent_post_ids: '', created_at: Time.utc(2024))
     variant = ActiveStorage::VariantRecord.create!(blob:, variation_digest: 'spec-variation')
     derived = ActiveStorage::Blob.create_before_direct_upload!(filename: 'derived.png',
@@ -78,7 +84,9 @@ RSpec.describe 'Counts artifact policy and external references' do
 
   it 'does not expose a private linked tag name through an external history snapshot' do
     tag = create(:tag, primary_name: 'private-linked-name')
-    TagVersion.create!(tag:, version_no: 1, event_type: 'discard', name: 'private-linked-name',
+    TagVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        tag:, version_no: 1, event_type: 'discard', name: 'private-linked-name',
         category: 'general', aliases: '', parent_tag_ids: '', created_at: Time.utc(2024),
         names: { ja: { name: 'private-linked-name', script_code: 'Jpan',
                        auto_generated: false, aliases: [] } })
@@ -87,7 +95,9 @@ RSpec.describe 'Counts artifact policy and external references' do
         name: 'nico:public-external', linked_tags: 'private-linked-name',
         created_at: Time.utc(2024))
     post = create(:post)
-    snapshot = PostVersion.create!(post:, version_no: 1, event_type: 'create',
+    snapshot = PostVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        post:, version_no: 1, event_type: 'create',
         title: 'external reference', url: post.url, tags: '',
         tags_json: [{ external_tag_id: external.id }], parent_post_ids: '',
         created_at: Time.utc(2024, 2))
@@ -112,13 +122,17 @@ RSpec.describe 'Counts artifact policy and external references' do
     child = create(:tag, primary_name: 'referencing child')
     [[parent, 'discard', '', 'private parent'],
      [child, 'create', parent.id.to_s, 'referencing child']].each do |tag, event, parents, name|
-      TagVersion.create!(tag:, version_no: 1, event_type: event, name:,
+      TagVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          tag:, version_no: 1, event_type: event, name:,
           category: 'general', aliases: '', parent_tag_ids: parents, created_at: Time.utc(2023),
           names: { ja: { name:, script_code: 'Jpan',
                          auto_generated: false, aliases: [] } })
     end
     post = create(:post)
-    row = PostVersion.create!(post:, version_no: 1, event_type: 'create', title: 'snapshot',
+    row = PostVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        post:, version_no: 1, event_type: 'create', title: 'snapshot',
         url: post.url, tags: '', parent_post_ids: '',
         tags_json: [{ tag_id: child.id, version_no: 1, name: 'referencing child', sections: [] }],
         created_at: Time.utc(2024))
diff --git a/backend/spec/models/locale_activation_spec.rb b/backend/spec/models/locale_activation_spec.rb
index e7d80d5..ac14678 100644
--- a/backend/spec/models/locale_activation_spec.rb
+++ b/backend/spec/models/locale_activation_spec.rb
@@ -23,9 +23,22 @@ RSpec.describe 'Locale activation', type: :model do
         .and_raise('backfill failed')
   end
 
+  it 'groups all tag histories from locale creation into one event' do
+    previous_ids = TagVersion.pluck(:id)
+
+    expect { build_locale.save! }.to change { event_rows.length }.by(1)
+
+    event = event_rows.last
+    expect(event).to include('event_type' => 'locales.create', 'created_by_user_id' => nil)
+    versions = TagVersion.where.not(id: previous_ids)
+    expect(versions.pluck(:tag_id)).to include(first.id, second.id)
+    expect(versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
+  end
+
   it 'rolls back a newly created locale and the preceding generated names' do
     locale = build_locale
     fail_second_generation!
+    events = event_rows
 
     expect { locale.save! }.to raise_error('backfill failed')
 
@@ -33,6 +46,7 @@ RSpec.describe 'Locale activation', type: :model do
     expect(generated_names).to be_empty
     expect(first.reload.name('ja')).to eq('activation_first')
     expect(second.reload.name('ja')).to eq('activation_second')
+    expect(event_rows).to eq(events)
   end
 
   it 'does not backfill while the locale is inactive' do
@@ -45,7 +59,14 @@ RSpec.describe 'Locale activation', type: :model do
     locale = build_locale(deprecated_at: Time.current)
     locale.save!
 
-    locale.update!(deprecated_at: nil)
+    previous_ids = TagVersion.pluck(:id)
+    expect { locale.update!(deprecated_at: nil) }.to change { event_rows.length }.by(1)
+
+    event = event_rows.last
+    expect(event).to include('event_type' => 'locales.restore', 'created_by_user_id' => nil)
+    versions = TagVersion.where.not(id: previous_ids)
+    expect(versions.pluck(:tag_id)).to include(first.id, second.id)
+    expect(versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
 
     expect(generated_names.pluck(:tag_id)).to contain_exactly(first.id, second.id)
     expect(generated_names).to all(have_attributes(
@@ -57,11 +78,13 @@ RSpec.describe 'Locale activation', type: :model do
     locale.save!
     previous_deprecated_at = locale.reload.deprecated_at
     fail_second_generation!
+    events = event_rows
 
     expect { locale.update!(deprecated_at: nil) }.to raise_error('backfill failed')
 
     expect(locale.reload.deprecated_at).to eq(previous_deprecated_at)
     expect(generated_names).to be_empty
+    expect(event_rows).to eq(events)
   end
 
   it 'preserves a human primary through deactivation and repeated activation' do
diff --git a/backend/spec/models/post_version_spec.rb b/backend/spec/models/post_version_spec.rb
index f29b530..e7b4031 100644
--- a/backend/spec/models/post_version_spec.rb
+++ b/backend/spec/models/post_version_spec.rb
@@ -12,20 +12,20 @@ RSpec.describe PostVersion, type: :model do
 
   let!(:post_version) do
     PostVersion.create!(
-      post: post_record,
-      version_no: 1,
-      event_type: 'create',
-      title: post_record.title,
-      url: post_record.url,
-      thumbnail_base: post_record.thumbnail_base,
-      tags: post_record.snapshot_tag_names.join(' '),
-      tags_json: post_record.snapshot_tags_json,
-      parent_post_ids: post_record.snapshot_parent_post_ids.join(' '),
-      original_created_from: post_record.original_created_from,
-      original_created_before: post_record.original_created_before,
-      created_at: Time.current,
-      created_by_user: nil
-    )
+        event_id: create_event!(event_type: 'unknown').id,
+        post: post_record,
+        version_no: 1,
+        event_type: 'create',
+        title: post_record.title,
+        url: post_record.url,
+        thumbnail_base: post_record.thumbnail_base,
+        tags: post_record.snapshot_tag_names.join(' '),
+        tags_json: post_record.snapshot_tags_json,
+        parent_post_ids: post_record.snapshot_parent_post_ids.join(' '),
+        original_created_from: post_record.original_created_from,
+        original_created_before: post_record.original_created_before,
+        created_at: Time.current,
+        created_by_user: nil)
   end
 
   it 'is read only after create' do
diff --git a/backend/spec/models/tag_name_sanitisation_rule_spec.rb b/backend/spec/models/tag_name_sanitisation_rule_spec.rb
index ab77c84..a0344f3 100644
--- a/backend/spec/models/tag_name_sanitisation_rule_spec.rb
+++ b/backend/spec/models/tag_name_sanitisation_rule_spec.rb
@@ -34,6 +34,31 @@ RSpec.describe TagNameSanitisationRule, type: :model do
       described_class.create!(priority: 10, source_pattern: 'foo_', replacement: 'foo')
     end
 
+    it 'groups renames, merges and affected post histories into one event' do
+      target = create(:tag, primary_name: 'foobar')
+      source = create(:tag, primary_name: 'temporary_source')
+      renamed = create(:tag, primary_name: 'temporary_rename')
+      source.tag_name('ja').update_columns(name: 'foo_bar')
+      renamed.tag_name('ja').update_columns(name: 'foo_baz')
+      post = create(:post)
+      PostTag.create!(post:, tag: source)
+      previous_tag_ids = TagVersion.pluck(:id)
+      previous_post_ids = PostVersion.pluck(:id)
+
+      expect { described_class.apply! }.to change { event_rows.length }.by(1)
+
+      event = event_rows.last
+      expect(event).to include(
+          'event_type' => 'tag_name_sanitisation_rules.apply', 'created_by_user_id' => nil)
+      tag_versions = TagVersion.where.not(id: previous_tag_ids)
+      expect(tag_versions.pluck(:tag_id).uniq).to contain_exactly(target.id, source.id, renamed.id)
+      expect(tag_versions.pluck(:event_type)).to include('discard', 'update')
+      expect(tag_versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
+      post_versions = PostVersion.where.not(id: previous_post_ids)
+      expect(post_versions.pluck(:post_id).uniq).to eq([post.id])
+      expect(post_versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
+    end
+
     [:primary, :alias].each do |source_kind|
       it "keeps the primary identity when a #{ source_kind } collides within its Tag" do
         tag = create(:tag, primary_name: source_kind == :primary ? 'temporary' : 'foobar')
@@ -83,6 +108,7 @@ RSpec.describe TagNameSanitisationRule, type: :model do
       PostTag.create!(post:, tag: source)
       models = [Tag, TagName, TagVersion, TagDescription, TagDescriptionVersion, PostTag]
       snapshots = models.map { |model| model.order(*Array(model.primary_key)).map(&:attributes) }
+      events = event_rows
 
       expect { described_class.apply! }.to raise_error(ActiveRecord::RecordInvalid)
 
@@ -90,6 +116,7 @@ RSpec.describe TagNameSanitisationRule, type: :model do
         model.order(*Array(model.primary_key)).map(&:attributes)
       end
       expect(after_state).to eq(snapshots)
+      expect(event_rows).to eq(events)
       expect(post.reload.tags).to contain_exactly(source)
       expect(target.reload.name('ja')).to eq('foobar')
     end
diff --git a/backend/spec/models/tag_spec.rb b/backend/spec/models/tag_spec.rb
index a247f93..b27f07e 100644
--- a/backend/spec/models/tag_spec.rb
+++ b/backend/spec/models/tag_spec.rb
@@ -665,20 +665,20 @@ RSpec.describe Tag, type: :model do
 
     def create_post_version_for!(post, version_no: 1, event_type: 'create', created_by_user: nil)
       PostVersion.create!(
-        post: post,
-        version_no: version_no,
-        event_type: event_type,
-        title: post.title,
-        url: post.url,
-        thumbnail_base: post.thumbnail_base,
-        tags: snapshot_tags(post),
-        tags_json: post.snapshot_tags_json,
-        parent_post_ids: post.snapshot_parent_post_ids.join(' '),
-        original_created_from: post.original_created_from,
-        original_created_before: post.original_created_before,
-        created_at: Time.current,
-        created_by_user: created_by_user
-      )
+          event_id: create_event!(event_type: 'unknown').id,
+          post: post,
+          version_no: version_no,
+          event_type: event_type,
+          title: post.title,
+          url: post.url,
+          thumbnail_base: post.thumbnail_base,
+          tags: snapshot_tags(post),
+          tags_json: post.snapshot_tags_json,
+          parent_post_ids: post.snapshot_parent_post_ids.join(' '),
+          original_created_from: post.original_created_from,
+          original_created_before: post.original_created_before,
+          created_at: Time.current,
+          created_by_user: created_by_user)
     end
 
     context 'when post versions are enabled' do
diff --git a/backend/spec/models/version_record_spec.rb b/backend/spec/models/version_record_spec.rb
index 561c19b..148859b 100644
--- a/backend/spec/models/version_record_spec.rb
+++ b/backend/spec/models/version_record_spec.rb
@@ -6,17 +6,18 @@ RSpec.describe VersionRecord, type: :model do
 
   it 'makes TagVersion read only after create' do
     version = TagVersion.create!(
-      tag: tag,
-      version_no: 1,
-      event_type: 'create',
-      name: primary_tag_name_for(tag, 'ja').name,
-      category: tag.category,
-      aliases: '',
-      names: { ja: { name: primary_tag_name_for(tag, 'ja').name,
-                     script_code: 'Jpan', auto_generated: false, aliases: [] } },
-      parent_tag_ids: '',
-      created_at: Time.current,
-      created_by_user: nil)
+        event_id: create_event!(event_type: 'unknown').id,
+        tag: tag,
+        version_no: 1,
+        event_type: 'create',
+        name: primary_tag_name_for(tag, 'ja').name,
+        category: tag.category,
+        aliases: '',
+        names: { ja: { name: primary_tag_name_for(tag, 'ja').name,
+                       script_code: 'Jpan', auto_generated: false, aliases: [] } },
+        parent_tag_ids: '',
+        created_at: Time.current,
+        created_by_user: nil)
 
     expect {
       version.update!(name: 'changed')
@@ -25,17 +26,18 @@ RSpec.describe VersionRecord, type: :model do
 
   it 'prevents TagVersion destroy' do
     version = TagVersion.create!(
-      tag: tag,
-      version_no: 1,
-      event_type: 'create',
-      name: primary_tag_name_for(tag, 'ja').name,
-      category: tag.category,
-      aliases: '',
-      names: { ja: { name: primary_tag_name_for(tag, 'ja').name,
-                     script_code: 'Jpan', auto_generated: false, aliases: [] } },
-      parent_tag_ids: '',
-      created_at: Time.current,
-      created_by_user: nil)
+        event_id: create_event!(event_type: 'unknown').id,
+        tag: tag,
+        version_no: 1,
+        event_type: 'create',
+        name: primary_tag_name_for(tag, 'ja').name,
+        category: tag.category,
+        aliases: '',
+        names: { ja: { name: primary_tag_name_for(tag, 'ja').name,
+                       script_code: 'Jpan', auto_generated: false, aliases: [] } },
+        parent_tag_ids: '',
+        created_at: Time.current,
+        created_by_user: nil)
 
     expect {
       version.destroy!
diff --git a/backend/spec/rails_helper.rb b/backend/spec/rails_helper.rb
index 8fae8f1..0b2f5c7 100644
--- a/backend/spec/rails_helper.rb
+++ b/backend/spec/rails_helper.rb
@@ -44,6 +44,7 @@ RSpec.configure do |config|
   end
 
   config.include TestRecords
+  config.include EventRecords
   config.include RakeTaskHelper
 
   # FactoryBot の create / create_list を使へるやぅにする
diff --git a/backend/spec/requests/counts_download_spec.rb b/backend/spec/requests/counts_download_spec.rb
index 5734869..7f0c224 100644
--- a/backend/spec/requests/counts_download_spec.rb
+++ b/backend/spec/requests/counts_download_spec.rb
@@ -84,7 +84,9 @@ RSpec.describe 'Counts native GET acknowledgement', type: :request do
     expect(response).to have_http_status(:not_found)
     job, file, attempt = ready_job('json')
     record = create(:post)
-    PostVersion.create!(post: record, version_no: 1, event_type: 'discard', title: 'private',
+    PostVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        post: record, version_no: 1, event_type: 'discard', title: 'private',
         url: record.url, tags: '', tags_json: [], parent_post_ids: '', created_at: Time.utc(2024))
     get "/counts/export/files/#{ file }", params: { job:, attempt: }
     expect(response).to have_http_status(:not_found)
diff --git a/backend/spec/requests/counts_history_boundaries_spec.rb b/backend/spec/requests/counts_history_boundaries_spec.rb
index 0bdb514..0335ff8 100644
--- a/backend/spec/requests/counts_history_boundaries_spec.rb
+++ b/backend/spec/requests/counts_history_boundaries_spec.rb
@@ -8,7 +8,9 @@ RSpec.describe 'History boundaries shared with Counts', type: :request do
 
   before do
     ['create', 'discard'].each_with_index do |event, index|
-      TagDescriptionVersion.create!(tag_id: tag.id, language_code: 'ja',
+      TagDescriptionVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          tag_id: tag.id, language_code: 'ja',
           version_no: index + 1, event_type: event, body: 'private body',
           created_at: Time.utc(2024))
     end
diff --git a/backend/spec/requests/counts_spec.rb b/backend/spec/requests/counts_spec.rb
index 0026d49..7dd78ff 100644
--- a/backend/spec/requests/counts_spec.rb
+++ b/backend/spec/requests/counts_spec.rb
@@ -10,12 +10,16 @@ RSpec.describe 'Legacy Counts API', type: :request do
                  category: 'general', sections: [] }
     [1, 2, 3].each do |number|
       time = Time.utc(2024, number == 3 ? 2 : 1, number, 0, 0)
-      PostVersion.create!(post:, version_no: number,
+      PostVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          post:, version_no: number,
           event_type: number == 1 ? 'create' : 'update',
           tags: '', url: post.url, title: "Snapshot #{ number }", parent_post_ids: '',
           tags_json: number == 1 ? [snapshot] : [],
           created_by_user: number == 3 ? nil : actor, created_at: time)
-      TagVersion.create!(tag:, version_no: number,
+      TagVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          tag:, version_no: number,
           event_type: number == 1 ? 'create' : 'update',
           name: 'counts tag', category: 'general', aliases: '', parent_tag_ids: '',
           names: { ja: { name: 'counts tag', script_code: 'Jpan',
diff --git a/backend/spec/requests/counts_table_spec.rb b/backend/spec/requests/counts_table_spec.rb
index 5e13aac..e5f9b26 100644
--- a/backend/spec/requests/counts_table_spec.rb
+++ b/backend/spec/requests/counts_table_spec.rb
@@ -14,7 +14,9 @@ RSpec.describe 'Counts table API', type: :request do
   end
 
   before do
-    TagVersion.create!(tag:, version_no: 1, event_type: 'create', name: 'alpha',
+    TagVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        tag:, version_no: 1, event_type: 'create', name: 'alpha',
         category: 'general', aliases: '', parent_tag_ids: '',
         names: { ja: { name: 'alpha', script_code: 'Jpan', auto_generated: false,  aliases: [] } },
         created_at: Time.utc(2023))
@@ -24,7 +26,9 @@ RSpec.describe 'Counts table API', type: :request do
         { tag_id: tag.id, version_no: 1, name: 'alpha', category: 'general', sections: [] },
         { external_tag_id: external.id }]
     [1, 2, 3].each do |number|
-      PostVersion.create!(post: record, version_no: number,
+      PostVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          post: record, version_no: number,
           event_type: number == 1 ? 'create' : 'update', url: record.url,
           title: "snapshot #{ number }", tags: '', parent_post_ids: '',
           tags_json: snapshots.first(number == 1 ? 2 : (number == 2 ? 1 : 0)),
@@ -134,7 +138,9 @@ RSpec.describe 'Counts table API', type: :request do
   end
 
   it 'checks the current latest flag beyond the execution timestamp' do
-    PostVersion.create!(post: record, version_no: 4, event_type: 'update',
+    PostVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        post: record, version_no: 4, event_type: 'update',
         url: record.url, title: 'after execution', tags: '', tags_json: [],
         parent_post_ids: '', created_at: Time.utc(2025, 2, 1))
     result = query
@@ -188,12 +194,16 @@ RSpec.describe 'Counts table API', type: :request do
       tag.tag_names.find_by!(language_code: 'ja', primary_flg: true)
           .update!(name: 'renamed-alpha')
       create(:tag_name, :alias, tag:, name: 'alpha-alias')
-      TagVersion.create!(tag:, version_no: 2, event_type: 'update', name: 'renamed-alpha',
+      TagVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          tag:, version_no: 2, event_type: 'update', name: 'renamed-alpha',
           category: 'general', aliases: 'alpha-alias', parent_tag_ids: '',
           names: { ja: { name: 'renamed-alpha', script_code: 'Jpan', auto_generated: false,
                          aliases: [{ name: 'alpha-alias', script_code: 'Jpan' }] } },
           created_at: Time.utc(2024, 2, 1))
-      PostVersion.create!(post: record, version_no: 4, event_type: 'update',
+      PostVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          post: record, version_no: 4, event_type: 'update',
           url: record.url, title: 'after rename', tags: '', parent_post_ids: '',
           tags_json: [{ tag_id: tag.id, version_no: 2, name: 'renamed-alpha',
                         category: 'general', sections: [] }],
@@ -257,7 +267,9 @@ RSpec.describe 'Counts table API', type: :request do
 
   it 'reports column overflow separately from malformed input' do
     101.times do |index|
-      PostVersion.create!(post: record, version_no: index + 4,
+      PostVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          post: record, version_no: index + 4,
           event_type: 'update', url: record.url, title: "column #{ index }",
           tags: '', tags_json: [], parent_post_ids: '', created_at: Time.utc(2024, 3))
     end
@@ -301,7 +313,9 @@ RSpec.describe 'Counts table API', type: :request do
         sign_in_as(create(:user, role)) if role
         # Equal timestamps deliberately require version ordering, not a date comparison.
         ['discard', 'restore', 'discard', 'restore'].each_with_index do |event, index|
-          PostVersion.create!(post: record, version_no: index + 4, event_type: event,
+          PostVersion.create!(
+              event_id: create_event!(event_type: 'unknown').id,
+              post: record, version_no: index + 4, event_type: event,
               url: record.url, title: event == 'restore' ? 'restored' : 'private snapshot',
               tags: '', tags_json: [], parent_post_ids: '', created_at: Time.utc(2024, 3))
         end
@@ -329,7 +343,9 @@ RSpec.describe 'Counts table API', type: :request do
       end
 
       it 'does not resolve private condition names even when the result set is empty' do
-        TagVersion.create!(tag:, version_no: 2, event_type: 'discard', name: 'secret tag',
+        TagVersion.create!(
+            event_id: create_event!(event_type: 'unknown').id,
+            tag:, version_no: 2, event_type: 'discard', name: 'secret tag',
             category: 'general', aliases: '', parent_tag_ids: '',
             names: { ja: { name: 'secret tag', script_code: 'Jpan', auto_generated: false, aliases: [] } },
             created_at: Time.utc(2024, 2))
@@ -345,7 +361,9 @@ RSpec.describe 'Counts table API', type: :request do
     end
   end
   it 'includes private history in the public timeline without an exclusion count' do
-    PostVersion.create!(post: record, version_no: 4, event_type: 'discard', url: record.url,
+    PostVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        post: record, version_no: 4, event_type: 'discard', url: record.url,
         title: 'hidden title', tags: '', tags_json: [], parent_post_ids: '',
         created_at: Time.utc(2024, 3))
     result = query(mode: 'summary', rows: 'created_at:month', cols: 'event_type')
diff --git a/backend/spec/requests/materials_spec.rb b/backend/spec/requests/materials_spec.rb
index 093a8f1..3a7f0cd 100644
--- a/backend/spec/requests/materials_spec.rb
+++ b/backend/spec/requests/materials_spec.rb
@@ -314,6 +314,7 @@ RSpec.describe 'Materials API', type: :request do
           .and change(Tag, :count).by(1)
           .and change(TagName, :count).by(1)
           .and change(MaterialVersion, :count).by(1)
+          .and change { event_rows.length }.by(1)
 
         expect(response).to have_http_status(:created)
 
@@ -325,6 +326,12 @@ RSpec.describe 'Materials API', type: :request do
         expect(material.file.attached?).to be(true)
         expect(material.version_no).to eq(1)
         expect(material.material_versions.first.event_type).to eq('create')
+        event = event_rows.last
+        expect(event).to include(
+            'event_type' => 'materials.create',
+            'created_by_user_id' => member_user.id)
+        expect(material.material_versions.first.event_id).to eq(event.fetch('id'))
+        expect(material.tag.tag_versions.pluck(:event_id)).to eq([event.fetch('id')])
         expect(material.material_export_items.first.export_path).to eq('伊地知ニジカ/created.png')
         expect(material.material_versions.first.export_paths_json).to eq(
           'legacy_drive' => '伊地知ニジカ/created.png'
@@ -509,6 +516,7 @@ RSpec.describe 'Materials API', type: :request do
             export_paths: { legacy_drive: '伊地知ニジカ/updated.jpg' }
           }
         end.to change(MaterialVersion, :count).by(2)
+            .and change { event_rows.length }.by(1)
 
         expect(response).to have_http_status(:ok)
 
@@ -517,6 +525,12 @@ RSpec.describe 'Materials API', type: :request do
         expect(material.tag.category).to eq('material')
         expect(material.url).to eq('https://example.com/updated-source')
         expect(material.updated_by_user).to eq(member_user)
+        event = event_rows.last
+        expect(event).to include(
+            'event_type' => 'materials.update',
+            'created_by_user_id' => member_user.id)
+        expect(material.material_versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
+        expect(material.tag.tag_versions.pluck(:event_id)).to eq([event.fetch('id')])
         expect(material.file.attached?).to be(true)
         expect(material.file.blob.id).not_to eq(old_blob_id)
         expect(ActiveStorage::Blob.where(id: old_blob_id).exists?).to be(true)
@@ -632,6 +646,7 @@ RSpec.describe 'Materials API', type: :request do
         MaterialVersionRecorder.record!(material:, event_type: :create,
                                         created_by_user: member_user)
         TagVersioning.ensure_snapshot!(tag, created_by_user: member_user)
+        events = event_rows
 
         expect do
           put "/materials/#{ material.id }", params: {
@@ -643,6 +658,7 @@ RSpec.describe 'Materials API', type: :request do
 
         expect(response).to have_http_status(:ok)
         expect(material.reload.version_no).to eq(1)
+        expect(event_rows).to eq(events)
       end
 
       it 'records update version when only export_path changes' do
@@ -807,6 +823,12 @@ RSpec.describe 'Materials API', type: :request do
 
     it 'creates source suppression as member' do
       sign_in_as(member_user)
+      materials = ['a', 'b'].map do |name|
+        Material.create!(
+            source_kind: 'google_drive_file', source_file_id: "blocked-#{ name }",
+            source_path: "伊地知ニジカ/危険素材/#{ name }.png",
+            url: "https://example.com/blocked-#{ name }", created_by_user: member_user)
+      end
 
       expect do
         post '/materials/suppressions', params: {
@@ -815,6 +837,7 @@ RSpec.describe 'Materials API', type: :request do
           reason: 'copyright_high_risk'
         }
       end.to change(MaterialSyncSuppression, :count).by(1)
+          .and change { event_rows.length }.by(1)
 
       expect(response).to have_http_status(:created)
       expect(json).to include(
@@ -822,6 +845,14 @@ RSpec.describe 'Materials API', type: :request do
         'drive_path' => '伊地知ニジカ/危険素材',
         'normalized_source_key' => 'google_drive_path_prefix:伊地知ニジカ/危険素材'
       )
+      event = event_rows.last
+      expect(event).to include(
+          'event_type' => 'materials.block', 'created_by_user_id' => member_user.id)
+      expect(materials.map { _1.reload.discarded? }).to eq([true, true])
+      versions = MaterialVersion.where(material_id: materials.map(&:id))
+      expect(versions.where(event_type: 'discard').pluck(:material_id))
+          .to match_array(materials.map(&:id))
+      expect(versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
     end
   end
 
@@ -860,11 +891,19 @@ RSpec.describe 'Materials API', type: :request do
       end
 
       it 'discards the material and returns 204' do
-        delete "/materials/#{ material.id }"
+        expect { delete "/materials/#{ material.id }" }
+            .to change { event_rows.length }.by(1)
 
         expect(response).to have_http_status(:no_content)
         expect(Material.find_by(id: material.id)).to be_nil
         expect(Material.with_discarded.find(material.id)).to be_discarded
+        event = event_rows.last
+        expect(event).to include(
+            'event_type' => 'materials.destroy',
+            'created_by_user_id' => member_user.id)
+        versions = MaterialVersion.where(material_id: material.id)
+        expect(versions.order(:version_no).last.event_type).to eq('discard')
+        expect(versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
       end
     end
   end
diff --git a/backend/spec/requests/nico_tags_spec.rb b/backend/spec/requests/nico_tags_spec.rb
index a554d26..fb6366a 100644
--- a/backend/spec/requests/nico_tags_spec.rb
+++ b/backend/spec/requests/nico_tags_spec.rb
@@ -177,6 +177,7 @@ RSpec.describe 'NicoTags', type: :request do
         }
       }.to change(TagVersion, :count).by(2)
        .and change(NicoTagVersion, :count).by(1)
+       .and change { event_rows.length }.by(1)
 
       expect(response).to have_http_status(:ok)
 
@@ -186,6 +187,10 @@ RSpec.describe 'NicoTags', type: :request do
       linked_versions = TagVersion.where(tag: [linked_a, linked_b]).order(:tag_id)
       expect(linked_versions.map(&:event_type)).to eq(['create', 'create'])
       expect(linked_versions.map(&:created_by_user_id)).to all(eq(admin.id))
+      event = event_rows.last
+      expect(event).to include(
+          'event_type' => 'tags.update', 'created_by_user_id' => admin.id)
+      expect(linked_versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
 
       versions = nico_tag.reload.nico_tag_versions.order(:version_no)
       expect(versions.map(&:event_type)).to eq(['create', 'update'])
diff --git a/backend/spec/requests/posts_spec.rb b/backend/spec/requests/posts_spec.rb
index f0d0ab7..bb05526 100644
--- a/backend/spec/requests/posts_spec.rb
+++ b/backend/spec/requests/posts_spec.rb
@@ -50,6 +50,8 @@ RSpec.describe 'Posts API', type: :request do
             post:,
             version_no: 1,
             event_type: 'create',
+            event_id: create_event!(
+                event_type: 'posts.create', created_by_user: post.uploaded_user).id,
             title: post.title,
             url: post.url,
             thumbnail_base: post.thumbnail_base,
@@ -1027,7 +1029,7 @@ RSpec.describe 'Posts API', type: :request do
 
     it 'dry-runs without persisting posts or new tags' do
       sign_in_as(member)
-      counts = [Post.count, Tag.count, TagName.count]
+      counts = [Post.count, Tag.count, TagName.count, event_rows.length]
 
       post '/posts?dry=1', params: post_write_params(
           title: 'dry-run post',
@@ -1043,7 +1045,7 @@ RSpec.describe 'Posts API', type: :request do
           [{ 'name' => 'new_dry_run_tag',
              'category' => 'character',
              'section_literals' => [] }])
-      expect([Post.count, Tag.count, TagName.count]).to eq(counts)
+      expect([Post.count, Tag.count, TagName.count, event_rows.length]).to eq(counts)
     end
 
     it 'dry-runs an existing URL without validating its upload' do
@@ -2218,20 +2220,20 @@ RSpec.describe 'Posts API', type: :request do
 
     def create_post_version! post, version_no:, event_type:, created_by_user:, created_at:
       PostVersion.create!(
-        post:,
-        version_no:,
-        event_type:,
-        title: post.title,
-        url: post.url,
-        thumbnail_base: post.thumbnail_base,
-        tags: snapshot_tags(post),
-        tags_json: post.snapshot_tags_json,
-        parent_post_ids: post.snapshot_parent_post_ids.join(' '),
-        original_created_from: post.original_created_from,
-        original_created_before: post.original_created_before,
-        created_at:,
-        created_by_user:
-      )
+          event_id: create_event!(event_type: 'unknown').id,
+          post:,
+          version_no:,
+          event_type:,
+          title: post.title,
+          url: post.url,
+          thumbnail_base: post.thumbnail_base,
+          tags: snapshot_tags(post),
+          tags_json: post.snapshot_tags_json,
+          parent_post_ids: post.snapshot_parent_post_ids.join(' '),
+          original_created_from: post.original_created_from,
+          original_created_before: post.original_created_before,
+          created_at:,
+          created_by_user:)
     end
 
     let!(:v1) do
@@ -2559,6 +2561,7 @@ RSpec.describe 'Posts API', type: :request do
             tags: 'spec_tag',
             thumbnail: dummy_upload)
       end.to change(PostVersion, :count).by(1)
+          .and change { event_rows.length }.by(1)
 
       expect(response).to have_http_status(:created)
 
@@ -2570,6 +2573,13 @@ RSpec.describe 'Posts API', type: :request do
       expect(version.url).to eq('https://example.com/versioned-post')
       expect(version.created_by_user_id).to eq(member.id)
       expect(version.tags).to eq(snapshot_tags(created_post))
+      event = event_rows.last
+      expect(event).to include(
+          'event_type' => 'posts.create', 'created_by_user_id' => member.id)
+      expect(version.event_id).to eq(event.fetch('id'))
+      tag_versions = TagVersion.where(tag_id: created_post.tags.select(:id))
+      expect(tag_versions).not_to be_empty
+      expect(tag_versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
     end
 
     it 'creates next version on PUT /posts/:id when snapshot changes' do
@@ -2585,6 +2595,7 @@ RSpec.describe 'Posts API', type: :request do
             title: 'updated title',
             tags: 'spec_tag_2')
       end.to change(PostVersion, :count).by(1)
+          .and change { event_rows.length }.by(1)
 
       expect(response).to have_http_status(:ok)
 
@@ -2594,6 +2605,12 @@ RSpec.describe 'Posts API', type: :request do
       expect(version.title).to eq('updated title')
       expect(version.created_by_user_id).to eq(member.id)
       expect(version.tags).to eq(snapshot_tags(post_record.reload))
+      event = event_rows.last
+      expect(event).to include(
+          'event_type' => 'posts.update', 'created_by_user_id' => member.id)
+      expect(version.event_id).to eq(event.fetch('id'))
+      tag_version = TagVersion.find_by!(tag_id: tag_name2.reload.tag_id, version_no: 1)
+      expect(tag_version.event_id).to eq(event.fetch('id'))
     end
 
     it 'does not create a new version on PUT /posts/:id when snapshot is unchanged' do
@@ -2602,6 +2619,7 @@ RSpec.describe 'Posts API', type: :request do
       PostTag.create!(post: post_record, tag: Tag.no_deerjikist)
       base_version = create_post_version_for!(post_record.reload)
 
+      events = event_rows
       expect {
         put "/posts/#{post_record.id}", params: post_write_params(
             base_version_no: base_version.version_no,
@@ -2615,10 +2633,12 @@ RSpec.describe 'Posts API', type: :request do
       expect(version.version_no).to eq(1)
       expect(version.event_type).to eq('create')
       expect(version.tags).to eq(snapshot_tags(post_record))
+      expect(event_rows).to eq(events)
     end
 
     it 'does not create a version when POST /posts is invalid' do
       sign_in_as(member)
+      events = event_rows
 
       expect do
         post '/posts', params: post_write_params(
@@ -2629,11 +2649,13 @@ RSpec.describe 'Posts API', type: :request do
       end.not_to change(PostVersion, :count)
 
       expect(response).to have_http_status(:unprocessable_entity)
+      expect(event_rows).to eq(events)
     end
 
     it 'does not create a version when PUT /posts/:id is invalid' do
       sign_in_as(member)
       base_version = create_post_version_for!(post_record)
+      events = event_rows
 
       expect do
         put "/posts/#{post_record.id}", params: post_write_params(
@@ -2645,6 +2667,7 @@ RSpec.describe 'Posts API', type: :request do
       end.not_to change(PostVersion, :count)
 
       expect(response).to have_http_status(:unprocessable_entity)
+      expect(event_rows).to eq(events)
     end
 
     it 'rejects second-bearing original created timestamps on POST /posts' do
diff --git a/backend/spec/requests/sitemap_entries_spec.rb b/backend/spec/requests/sitemap_entries_spec.rb
index 26d63cf..65ef340 100644
--- a/backend/spec/requests/sitemap_entries_spec.rb
+++ b/backend/spec/requests/sitemap_entries_spec.rb
@@ -119,7 +119,9 @@ RSpec.describe 'Sitemap entries API', type: :request do
       # Legacy empty bodies can exist even though the write service rejects them.
       empty = TagDescription.create!(tag: empty_tag, language_code: 'ain',
                                      body: '', version_no: 1)
-      TagDescriptionVersion.create!(tag_id: empty_tag.id, language_code: 'ain',
+      TagDescriptionVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          tag_id: empty_tag.id, language_code: 'ain',
                                     body: '', version_no: 1, event_type: 'create',
                                     created_by_user_id: user.id, created_at: Time.current)
 
diff --git a/backend/spec/requests/tag_children_spec.rb b/backend/spec/requests/tag_children_spec.rb
index dd22144..22365b4 100644
--- a/backend/spec/requests/tag_children_spec.rb
+++ b/backend/spec/requests/tag_children_spec.rb
@@ -53,8 +53,13 @@ RSpec.describe "TagChildren", type: :request do
         expect { do_request }
           .to change { parent.reload.children.ids.include?(child.id) }
           .from(false).to(true)
+          .and change { event_rows.length }.by(1)
 
         expect(response).to have_http_status(:no_content)
+        event = event_rows.last
+        expect(event).to include(
+            'event_type' => 'tags.update', 'created_by_user_id' => admin.id)
+        expect(child.tag_versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
       end
 
       it 'returns 422 and does not create relation when the new link makes a cycle' do
@@ -132,6 +137,7 @@ RSpec.describe "TagChildren", type: :request do
         expect {
           do_request
         }.to change(TagVersion, :count).by(2)
+            .and change { event_rows.length }.by(1)
 
         expect(response).to have_http_status(:no_content)
 
@@ -140,6 +146,10 @@ RSpec.describe "TagChildren", type: :request do
         expect(versions.first.parent_tag_ids.split).to include(parent.id.to_s)
         expect(versions.second.parent_tag_ids).to eq('')
         expect(versions.second.created_by_user_id).to eq(admin.id)
+        event = event_rows.last
+        expect(event).to include(
+            'event_type' => 'tags.update', 'created_by_user_id' => admin.id)
+        expect(versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
       end
     end
 
diff --git a/backend/spec/requests/tag_description_wiki_spec.rb b/backend/spec/requests/tag_description_wiki_spec.rb
index 19b0e19..0b19cb0 100644
--- a/backend/spec/requests/tag_description_wiki_spec.rb
+++ b/backend/spec/requests/tag_description_wiki_spec.rb
@@ -8,6 +8,7 @@ RSpec.describe 'Tag description Wiki compatibility', type: :request do
   let!(:versions) do
     ['A', 'A', 'new-version'].each_with_index.map do |body, index|
       TagDescriptionVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
           tag_id: tag.id, language_code: 'ja', version_no: index + 1,
           event_type: index.zero? ? 'create' : 'update', body:,
           created_by_user_id: user.id, created_at: Time.current)
@@ -73,6 +74,7 @@ RSpec.describe 'Tag description Wiki compatibility', type: :request do
     let!(:other_tag) { create(:tag, primary_name: 'other_description_contract') }
     let!(:other_version) do
       TagDescriptionVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
           tag_id: other_tag.id, language_code: 'ja', version_no: 1,
           event_type: 'create', body: 'other tag history',
           created_by_user_id: user.id, created_at: Time.current)
diff --git a/backend/spec/requests/tag_descriptions_spec.rb b/backend/spec/requests/tag_descriptions_spec.rb
index ceff8fd..ebd6ebe 100644
--- a/backend/spec/requests/tag_descriptions_spec.rb
+++ b/backend/spec/requests/tag_descriptions_spec.rb
@@ -13,7 +13,7 @@ RSpec.describe 'Tag description native API', type: :request do
   def stored_state
     [TagDescription, TagDescriptionVersion, TagName, TagVersion].map do |model|
       model.order(:id).map(&:attributes)
-    end
+    end + [event_rows]
   end
 
   def fail_save model
@@ -91,6 +91,8 @@ RSpec.describe 'Tag description native API', type: :request do
     let!(:version) do
       TagDescriptionVersion.create!(
           tag_id: tag.id, language_code: 'ja', body: 'native version', version_no: 1,
+          event_id: create_event!(
+              event_type: 'tag_descriptions.create', created_by_user: author).id,
           event_type: 'create', created_by_user_id: author.id, created_at: Time.current)
     end
     let(:name_endpoint) { "/tags/name/#{ tag_name.name }/description" }
@@ -185,6 +187,7 @@ RSpec.describe 'Tag description native API', type: :request do
         expect { native_write(body: 'created natively') }
             .to change(TagDescription, :count).by(1)
             .and change(TagDescriptionVersion, :count).by(1)
+            .and change { event_rows.length }.by(1)
         expect(response).to have_http_status(:created)
         description = TagDescription.find_by!(tag_id: tag.id, language_code: 'ja')
         expect_current(description)
@@ -192,6 +195,10 @@ RSpec.describe 'Tag description native API', type: :request do
         expect(version).to have_attributes(
             body: 'created natively', version_no: 1, event_type: 'create',
             created_by_user_id: editor.id)
+        event = event_rows.last
+        expect(event).to include(
+            'event_type' => 'tag_descriptions.create', 'created_by_user_id' => editor.id)
+        expect(version.event_id).to eq(event.fetch('id'))
         get "/wiki/#{ description.id }"
         expect(response).to have_http_status(:ok)
         expect(json).to include('body' => description.body, 'revision_id' => version.id)
@@ -223,6 +230,7 @@ RSpec.describe 'Tag description native API', type: :request do
         expect(description.version_no).to eq(1)
         expect { native_write(body: 'after') }
             .to change(TagDescriptionVersion, :count).by(1)
+            .and change { event_rows.length }.by(1)
         expect(response).to have_http_status(:ok)
         expect(description.reload).to have_attributes(
             body: 'after', version_no: 2,
@@ -234,6 +242,10 @@ RSpec.describe 'Tag description native API', type: :request do
         expect(version).to have_attributes(
             tag_id: tag.id, language_code: 'ja', version_no: 2, event_type: 'update',
             body: 'after', created_by_user_id: editor.id)
+        event = event_rows.last
+        expect(event).to include(
+            'event_type' => 'tag_descriptions.update', 'created_by_user_id' => editor.id)
+        expect(version.event_id).to eq(event.fetch('id'))
         expect(revision.body).to eq(version.body)
         expect(description_versions(page).order(:version_no).last.body).to eq(version.body)
         get "/wiki/#{ description.id }"
diff --git a/backend/spec/requests/tag_groups_spec.rb b/backend/spec/requests/tag_groups_spec.rb
index 091d919..62b7152 100644
--- a/backend/spec/requests/tag_groups_spec.rb
+++ b/backend/spec/requests/tag_groups_spec.rb
@@ -121,10 +121,14 @@ RSpec.describe 'Tag group native API', type: :request do
     [:create, :update].each do |operation|
       it "#{ operation }s structured data and keeps unplaced membership" do
         previous = group_version(group_id, 1)
-        native_write(operation)
+        expect { native_write(operation) }.to change { event_rows.length }.by(1)
         expect(response).to have_http_status(operation == :create ? :created : :ok)
         id = operation == :create ? json.fetch('id') : group_id
         number = operation == :create ? 1 : 2
+        event = event_rows.last
+        event_type = operation == :create ? 'tag_groups.create' : 'tag_groups.update'
+        expect(event).to include('event_type' => event_type, 'created_by_user_id' => user.id)
+        expect(group_version(id, number).fetch('event_id')).to eq(event.fetch('id'))
         expect_group_snapshot(
             id, number:, members: snapshot_members, placements:, description: '全体の説明')
         expect(group_version(group_id, 1)).to eq(previous)
diff --git a/backend/spec/requests/tag_versions_spec.rb b/backend/spec/requests/tag_versions_spec.rb
index f05bb0b..9eb88e9 100644
--- a/backend/spec/requests/tag_versions_spec.rb
+++ b/backend/spec/requests/tag_versions_spec.rb
@@ -30,6 +30,7 @@ RSpec.describe 'TagVersions API', type: :request do
   )
     version =
         TagVersion.create!(
+            event_id: create_event!(event_type: 'unknown').id,
             tag: tag,
             version_no: version_no,
             event_type: event_type,
diff --git a/backend/spec/requests/tags_spec.rb b/backend/spec/requests/tags_spec.rb
index 7a66889..7588257 100644
--- a/backend/spec/requests/tags_spec.rb
+++ b/backend/spec/requests/tags_spec.rb
@@ -705,6 +705,7 @@ RSpec.describe 'Tags API', type: :request do
         expect {
           patch "/tags/#{tag.id}", params: { name: 'new_tag_name', category: 'meme' }
         }.to change(TagVersion, :count).by(2)
+            .and change { event_rows.length }.by(1)
 
         expect(response).to have_http_status(:ok)
 
@@ -719,6 +720,10 @@ RSpec.describe 'Tags API', type: :request do
         expect(version_primary_name(versions.second, 'ja')).to eq('new_tag_name')
         expect(versions.second.category).to eq('meme')
         expect(versions.second.created_by_user_id).to eq(member_user.id)
+        event = event_rows.last
+        expect(event).to include(
+            'event_type' => 'tags.update', 'created_by_user_id' => member_user.id)
+        expect(versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
       end
 
       it 'updates deprecated state and records it in tag versions' do
@@ -1275,6 +1280,7 @@ RSpec.describe 'Tags API', type: :request do
             deprecated: '0',
           }
         }.to change(TagVersion, :count).by(2)
+            .and change { event_rows.length }.by(1)
 
         expect(response).to have_http_status(:ok)
 
@@ -1290,6 +1296,10 @@ RSpec.describe 'Tags API', type: :request do
         expect(versions.second.category).to eq('meta')
         expect(version_alias_names(versions.second, 'ja')).to include('spec_tag')
         expect(versions.second.created_by_user_id).to eq(member_user.id)
+        event = event_rows.last
+        expect(event).to include(
+            'event_type' => 'tags.update', 'created_by_user_id' => member_user.id)
+        expect(versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
       end
 
       it '同じ tag の旧 alias へ戻しても rename できる' do
diff --git a/backend/spec/services/counts_export_spec.rb b/backend/spec/services/counts_export_spec.rb
index de205d1..3e96153 100644
--- a/backend/spec/services/counts_export_spec.rb
+++ b/backend/spec/services/counts_export_spec.rb
@@ -143,7 +143,9 @@ RSpec.describe 'Counts export contracts' do
     expect(CountsExportDownload.find(token, 'owner')).not_to be_nil
     expect(CountsExportDownload.find(token, 'other')).to be_nil
     post = create(:post)
-    PostVersion.create!(post:, version_no: 1, event_type: 'discard', url: post.url,
+    PostVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        post:, version_no: 1, event_type: 'discard', url: post.url,
         title: 'private', tags: '', tags_json: [], parent_post_ids: '', created_at: Time.utc(2024))
     expect(CountsExportDownload.find(token, 'owner')).to be_nil
   end
diff --git a/backend/spec/services/counts_pivot_security_spec.rb b/backend/spec/services/counts_pivot_security_spec.rb
index 337d7a5..7245e43 100644
--- a/backend/spec/services/counts_pivot_security_spec.rb
+++ b/backend/spec/services/counts_pivot_security_spec.rb
@@ -13,7 +13,9 @@ RSpec.describe 'Counts masked pivot contracts' do
   let!(:tags) do
     ['A', 'B', 'C'].map do |name|
       tag = create(:tag, primary_name: name)
-      TagVersion.create!(tag:, version_no: 1, event_type: 'create', name:,
+      TagVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          tag:, version_no: 1, event_type: 'create', name:,
           category: 'general', aliases: '', parent_tag_ids: '',
           names: { ja: { name:, script_code: 'Jpan', auto_generated: false, aliases: [] } },
           created_at: Time.utc(2023))
@@ -25,12 +27,16 @@ RSpec.describe 'Counts masked pivot contracts' do
     [[0, 1], [0], [1, 2]].each_with_index do |indexes, index|
       post = create(:post)
       snapshot = indexes.map { { tag_id: tags[_1].id, version_no: 1, sections: [] } }
-      PostVersion.create!(post:, version_no: 1, event_type: 'create',
+      PostVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          post:, version_no: 1, event_type: 'create',
           title: "hidden-title-#{ index }", url: post.url, tags: '', tags_json: snapshot,
           parent_post_ids: '', created_by_user: member, created_at: Time.utc(2024, 1))
       next if index == 2
 
-      PostVersion.create!(post:, version_no: 2, event_type: 'discard',
+      PostVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          post:, version_no: 2, event_type: 'discard',
           title: "private-discard-#{ index }", url: post.url, tags: '', tags_json: [],
           parent_post_ids: '', created_by_user: member, created_at: Time.utc(2024, 2))
     end
@@ -98,12 +104,16 @@ RSpec.describe 'Counts masked pivot contracts' do
 
   it 'uses changed-record membership for totals and hides changes to private predecessors' do
     post = PostVersion.order(:id).last.post
-    PostVersion.create!(post:, version_no: 2, event_type: 'update', title: 'changed-public-title',
+    PostVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        post:, version_no: 2, event_type: 'update', title: 'changed-public-title',
         url: post.url, tags: '',
         tags_json: [{ tag_id: tags.last.id, version_no: 1, sections: [] }],
         parent_post_ids: '', created_at: Time.utc(2024, 3))
     restored = PostVersion.order(:id).first.post
-    PostVersion.create!(post: restored, version_no: 3, event_type: 'restore',
+    PostVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        post: restored, version_no: 3, event_type: 'restore',
         title: 'restored-public-title', url: restored.url, tags: '',
         tags_json: [{ tag_id: tags.first.id, version_no: 1, sections: [] }], parent_post_ids: '',
         created_at: Time.utc(2024, 4))
@@ -134,7 +144,9 @@ RSpec.describe 'Counts masked pivot contracts' do
 
   it 'keeps NULL as a public data value rather than treating it as a rollup' do
     post = create(:post)
-    PostVersion.create!(post:, version_no: 1, event_type: 'create', title: 'public bot',
+    PostVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
+        post:, version_no: 1, event_type: 'create', title: 'public bot',
         url: post.url, tags: '', tags_json: [], parent_post_ids: '', created_at: Time.utc(2024, 5))
     page = result(nil, 'rows' => 'user', 'cols' => 'event_type')
     expect(page[:cells].select { _1[:values].first.nil? }.sum { _1[:count] }).to eq(1)
@@ -145,7 +157,9 @@ RSpec.describe 'Counts masked pivot contracts' do
   it 'sorts public numeric values numerically alongside a masked group' do
     post = PostVersion.order(:id).last.post
     [2, 10].each do |number|
-      PostVersion.create!(post:, version_no: number, event_type: 'update',
+      PostVersion.create!(
+          event_id: create_event!(event_type: 'unknown').id,
+          post:, version_no: number, event_type: 'update',
           title: 'public numeric sort', url: post.url, tags: '', tags_json: [],
           parent_post_ids: '', created_at: Time.utc(2024, 5))
     end
diff --git a/backend/spec/services/material_sync_importer_spec.rb b/backend/spec/services/material_sync_importer_spec.rb
index b2734e3..bf1a774 100644
--- a/backend/spec/services/material_sync_importer_spec.rb
+++ b/backend/spec/services/material_sync_importer_spec.rb
@@ -56,6 +56,7 @@ RSpec.describe MaterialSyncImporter do
                                profile: 'legacy_drive',
                                export_path: '素材/a.png',
                                created_by_user: user)
+    events = event_rows
 
     result =
         described_class.import!(
@@ -73,6 +74,7 @@ RSpec.describe MaterialSyncImporter do
     expect(result.action).to eq(:unchanged)
     expect(material.reload.tag).to eq(tag)
     expect(material.url).to eq('https://example.com/human')
+    expect(event_rows).to eq(events)
   end
 
   it 'suppresses a Google Drive candidate by relative path prefix' do
diff --git a/backend/spec/services/material_sync_runner_spec.rb b/backend/spec/services/material_sync_runner_spec.rb
index ace4f45..810830a 100644
--- a/backend/spec/services/material_sync_runner_spec.rb
+++ b/backend/spec/services/material_sync_runner_spec.rb
@@ -14,6 +14,47 @@ RSpec.describe MaterialSyncRunner do
   let(:drive_client) { instance_double(GoogleDrive::ApiClient) }
 
   describe '#sync!' do
+    it 'groups created and updated materials across sync sources into one event' do
+      existing = Material.create!(
+          source_kind: 'google_drive_file', source_file_id: 'existing-file',
+          source_path: 'old.png', url: 'https://example.com/existing',
+          created_by_user: user, updated_by_user: user)
+      source
+      MaterialSyncSource.create!(
+          name: 'Second Drive source', source_kind: 'google_drive_path',
+          source_file_id: 'folder-456', profile: 'legacy_drive',
+          created_by_user: user, updated_by_user: user)
+      allow(GoogleDrive::ApiClient).to receive(:new).and_return(drive_client)
+      allow(drive_client).to receive(:each_material_file_under_folder) do |folder_id, &block|
+        id = folder_id == 'folder-123' ? 'existing-file' : 'new-file'
+        block.call(id:, name: "#{ id }.png", mime_type: 'image/png',
+                   relative_path: "素材/#{ id }.png", sha256_checksum: nil,
+                   web_view_link: "https://drive.google.com/file/d/#{ id }/view")
+      end
+      allow(drive_client).to receive(:download_to_tempfile) do |id, **|
+        Tempfile.new(['sync-event', '.png']).tap do |file|
+          file.binmode
+          file.write(id)
+          file.rewind
+        end
+      end
+      allow(MaterialThumbnailGenerator).to receive(:generate!)
+      previous_ids = MaterialVersion.pluck(:id)
+
+      result = nil
+      expect { result = described_class.sync_enabled! }
+          .to change { event_rows.length }.by(1)
+
+      expect(result).to have_attributes(imported: 1, updated: 1, failed: 0)
+      event = event_rows.last
+      expect(event.fetch('event_type')).to eq('materials.sync')
+      created = Material.find_by!(source_file_id: 'new-file')
+      versions = MaterialVersion.where.not(id: previous_ids)
+      expect(versions.pluck(:material_id).uniq).to contain_exactly(existing.id, created.id)
+      expect(versions.pluck(:event_type)).to include('create', 'update')
+      expect(versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
+    end
+
     it 'imports google drive path candidates as they are yielded' do
       first_entry = { id: 'file-1',
                       name: 'a.png',
diff --git a/backend/spec/services/post_bulk_creator_spec.rb b/backend/spec/services/post_bulk_creator_spec.rb
index 2976143..570e610 100644
--- a/backend/spec/services/post_bulk_creator_spec.rb
+++ b/backend/spec/services/post_bulk_creator_spec.rb
@@ -88,3 +88,86 @@ RSpec.describe PostBulkCreator do
     expect(creator).to have_received(:create!).with(locale).once
   end
 end
+
+RSpec.describe 'PostBulkCreator event grouping', type: :model do
+  self.use_transactional_tests = false
+
+  before do
+    expect(Rails.env.test?).to be(true)
+    expect(ApplicationRecord.connection.open_transactions).to eq(0)
+    @prefix = "bulk_event_#{ SecureRandom.hex(8) }"
+    @actor = create(:user, :member)
+    allow(PostCreatePreflight).to receive(:new) do |attributes:, **|
+      specs = [{ name: attributes.fetch(:title), category: :general }]
+      instance_double(PostCreatePreflight, run: attributes.merge(
+          existing_post_id: nil, snapshot_tag_specs: specs, post_tag_specs: specs,
+          normalised_parent_post_ids: [], tag_sections: { }, video_ms: nil))
+    end
+  end
+
+  after do
+    if @actor
+      posts = Post.where(uploaded_user: @actor)
+      PostTagSection.where(post_id: posts.select(:id)).delete_all
+      PostTag.where(post_id: posts.select(:id)).delete_all
+      PostVersion.where(post_id: posts.select(:id)).delete_all
+      posts.delete_all
+      tag_ids = TagName.where(name: ["#{ @prefix }_1", "#{ @prefix }_2"]).pluck(:tag_id)
+      TagVersion.where(tag_id: tag_ids).delete_all
+      TagName.where(tag_id: tag_ids).delete_all
+      Tag.where(id: tag_ids).delete_all
+      EventRecords::EventRow.where(created_by_user_id: @actor.id).delete_all
+      @actor.delete
+    end
+  end
+
+  it 'shares one creation event between real workers and their tag histories' do
+    posts = [1, 2].map do |number|
+      { title: "#{ @prefix }_#{ number }",
+        url: "https://example.com/#{ @prefix }/#{ number }" }
+    end
+
+    result = nil
+    expect {
+      result = PostBulkCreator.new(actor: @actor, posts:, thumbnails: { })
+          .run(Locale.nipponese)
+    }.to change { event_rows.length }.by(1)
+
+    expect(result.fetch(:results).map { _1.fetch(:status) }).to eq(['created', 'created'])
+    event = event_rows.last
+    expect(event).to include(
+        'event_type' => 'posts.create', 'created_by_user_id' => @actor.id)
+    versions = PostVersion.where(created_by_user_id: @actor.id)
+    expect(versions.count).to eq(2)
+    expect(versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
+    tag_versions = TagVersion.where(created_by_user_id: @actor.id)
+    expect(tag_versions.count).to eq(2)
+    expect(tag_versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
+  end
+
+  it 'does not leave an event when every row fails validation' do
+    preflight = instance_double(PostCreatePreflight)
+    allow(PostCreatePreflight).to receive(:new).and_return(preflight)
+    allow(preflight).to receive(:run).and_raise(
+        PostCreatePreflight::ValidationFailed.new(fields: { url: ['invalid'] }))
+    events = event_rows
+
+    result = PostBulkCreator.new(
+        actor: @actor, posts: [{ title: 'invalid' }], thumbnails: { }).run(Locale.nipponese)
+
+    expect(result.fetch(:results).map { _1.fetch(:status) }).to eq(['failed'])
+    expect(event_rows).to eq(events)
+  end
+
+  it 'does not leave an event when every row is skipped' do
+    preflight = instance_double(PostCreatePreflight, run: { existing_post_id: 123 })
+    allow(PostCreatePreflight).to receive(:new).and_return(preflight)
+    events = event_rows
+
+    result = PostBulkCreator.new(
+        actor: @actor, posts: [{ title: 'existing' }], thumbnails: { }).run(Locale.nipponese)
+
+    expect(result.fetch(:results).map { _1.fetch(:status) }).to eq(['skipped'])
+    expect(event_rows).to eq(events)
+  end
+end
diff --git a/backend/spec/services/shared_resource_id_allocator_mysql_spec.rb b/backend/spec/services/shared_resource_id_allocator_mysql_spec.rb
index 8487c30..e501c4e 100644
--- a/backend/spec/services/shared_resource_id_allocator_mysql_spec.rb
+++ b/backend/spec/services/shared_resource_id_allocator_mysql_spec.rb
@@ -16,6 +16,7 @@ RSpec.describe 'Shared resource IDs across MySQL connections', type: :model do
     @tag = create(:tag, primary_name: "#{ @prefix }_owner")
     @group = TagGroup.create!(name: "#{ @prefix }_owner", version_no: 1)
     @tag_ids = Queue.new
+    @event_ids = Queue.new
     @threads = []
     @release = Queue.new
   end
@@ -42,6 +43,9 @@ RSpec.describe 'Shared resource IDs across MySQL connections', type: :model do
       ids << @tag.id if @tag
       TagName.where(tag_id: ids).delete_all
       Tag.where(id: ids).delete_all
+      event_ids = []
+      event_ids << @event_ids.pop until @event_ids.empty? if @event_ids
+      EventRecords::EventRow.where(id: event_ids).delete_all
     end
   end
 
@@ -72,11 +76,17 @@ RSpec.describe 'Shared resource IDs across MySQL connections', type: :model do
     when :description
       TagDescription.create!(tag: @tag, language_code: 'ja', body: '', version_no: 1)
     when :description_version
+      event = create_event!(event_type: 'unknown')
+      @event_ids << event.id
       TagDescriptionVersion.create!(
+          event_id: event.id,
           tag_id: @tag.id, language_code: 'ja', body: 'body', version_no: 1,
           event_type: 'create', created_at: Time.current)
     when :group_version
+      event = create_event!(event_type: 'unknown')
+      @event_ids << event.id
       TagGroupVersion.create!(
+          event_id: event.id,
           tag_group: @group, name: @group.name, version_no: 1, event_type: 'create',
           members: [], placements: [], created_at: Time.current)
     when :group
diff --git a/backend/spec/services/shared_resource_id_allocator_spec.rb b/backend/spec/services/shared_resource_id_allocator_spec.rb
index 1fa3f18..773a453 100644
--- a/backend/spec/services/shared_resource_id_allocator_spec.rb
+++ b/backend/spec/services/shared_resource_id_allocator_spec.rb
@@ -15,10 +15,12 @@ RSpec.describe SharedResourceIdAllocator do
       TagGroup.new(id:, name: "group #{ number }", version_no: number)
     when 'tag_description_versions'
       TagDescriptionVersion.new(
+          event_id: create_event!(event_type: 'unknown').id,
           id:, tag_id: tag.id, language_code: 'ja', body: 'body', version_no: number,
           event_type: 'create', created_at: Time.current)
     when 'tag_group_versions'
       TagGroupVersion.new(
+          event_id: create_event!(event_type: 'unknown').id,
           id:, tag_group_id: group.id, name: group.name, version_no: number,
           event_type: 'create', members: [], placements: [], created_at: Time.current)
     end
@@ -82,6 +84,7 @@ RSpec.describe SharedResourceIdAllocator do
   it 'reserves a Tag ID referenced only by remaining description history' do
     high = [Tag.maximum(:id).to_i, ExternalTag.maximum(:id).to_i].max + 1000
     TagDescriptionVersion.create!(
+        event_id: create_event!(event_type: 'unknown').id,
         tag_id: high, language_code: 'ja', version_no: 1, body: 'history',
         event_type: 'create', created_at: Time.current)
     expect(create(:tag).id).to be > high
diff --git a/backend/spec/services/youtube/sync_spec.rb b/backend/spec/services/youtube/sync_spec.rb
index e8d7a24..0ed81e7 100644
--- a/backend/spec/services/youtube/sync_spec.rb
+++ b/backend/spec/services/youtube/sync_spec.rb
@@ -42,6 +42,41 @@ RSpec.describe Youtube::Sync do
   end
 
   describe '#sync!' do
+    it 'groups created and updated post histories into one sync event' do
+      allow(PostVersionRecorder).to receive(:record!).and_call_original
+      allow(PostVersionRecorder).to receive(:ensure_snapshot!).and_call_original
+      existing = create(:post, url: 'https://youtu.be/video-1', title: 'old title')
+      allow(sync).to receive(:query_terms).and_return([])
+      allow(sync).to receive(:playlist_ids).and_return(['PL123'])
+      allow(client).to receive(:playlist_items).with(playlist_id: 'PL123', page_token: nil)
+          .and_return('items' => [
+              { 'contentDetails' => { 'videoId' => 'video-1' } },
+              { 'contentDetails' => { 'videoId' => 'video-2' } }])
+      allow(client).to receive(:videos).with(['video-1', 'video-2'])
+          .and_return('items' => [
+              youtube_video_item(id: 'video-1', title: 'updated', channel_id: 'UC_NONE'),
+              youtube_video_item(id: 'video-2', title: 'created', channel_id: 'UC_NONE')])
+      previous_post_ids = PostVersion.pluck(:id)
+      previous_tag_ids = TagVersion.pluck(:id)
+
+      expect { sync.sync! }.to change { event_rows.length }.by(1)
+
+      event = event_rows.last
+      expect(event).to include('event_type' => 'posts.sync', 'created_by_user_id' => nil)
+      versions = PostVersion.where.not(id: previous_post_ids)
+      created = Post.find_by!(url: 'https://www.youtube.com/watch?v=video-2')
+      expect(versions.pluck(:post_id).uniq).to contain_exactly(existing.id, created.id)
+      expect(versions.pluck(:event_type)).to include('create', 'update')
+      expect(versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
+      tag_versions = TagVersion.where.not(id: previous_tag_ids)
+      expect(tag_versions).not_to be_empty
+      expect(tag_versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
+
+      events = event_rows
+      expect { sync.sync! }.not_to change(PostVersion, :count)
+      expect(event_rows).to eq(events)
+    end
+
     it 'returns without fetching video details when no video ids are discovered' do
       allow(sync).to receive(:query_terms).and_return([])
       allow(sync).to receive(:playlist_ids).and_return([])
diff --git a/backend/spec/support/event_records.rb b/backend/spec/support/event_records.rb
new file mode 100644
index 0000000..f68b151
--- /dev/null
+++ b/backend/spec/support/event_records.rb
@@ -0,0 +1,15 @@
+module EventRecords
+  # Read the database contract without requiring an application Event model.
+  class EventRow < ActiveRecord::Base
+    self.table_name = 'events'
+  end
+
+  def event_rows
+    EventRow.order(:id).map(&:attributes)
+  end
+
+  def create_event! event_type:, created_by_user: nil
+    EventRow.create!(event_type:, created_by_user_id: created_by_user&.id,
+                     created_at: Time.current)
+  end
+end
diff --git a/backend/spec/support/tag_group_records.rb b/backend/spec/support/tag_group_records.rb
index 82c1693..9023907 100644
--- a/backend/spec/support/tag_group_records.rb
+++ b/backend/spec/support/tag_group_records.rb
@@ -3,11 +3,13 @@ require 'cgi'
 # SQL fixtures use the migration contract without inventing application models.
 # Include explicitly; these helpers must not affect unrelated examples.
 module TagGroupRecords
+  include EventRecords
+
   GROUP_TABLES = [
       'tag_groups', 'tag_group_tags', 'tag_group_groups',
       'tag_group_placements', 'tag_group_versions'].freeze
   WRITE_TABLES = GROUP_TABLES + [
-      'tag_names', 'tag_description_versions',
+      'events', 'tag_names', 'tag_description_versions',
       'tag_descriptions']
 
   def group_connection = ApplicationRecord.connection
@@ -114,6 +116,7 @@ module TagGroupRecords
     group_id = insert_group_row('tag_groups', attributes)
     insert_group_row('tag_group_versions', {
         tag_group_id: group_id, version_no: 1, event_type: 'create',
+        event_id: create_event!(event_type: 'tag_groups.create', created_by_user: user).id,
         name:, description:, members: members.to_json, placements: placements.to_json,
         created_by_user_id: user.id, created_at: now })
     members.each do |member|
diff --git a/backend/spec/tasks/nico_sync_spec.rb b/backend/spec/tasks/nico_sync_spec.rb
index db6d55a..ab3be13 100644
--- a/backend/spec/tasks/nico_sync_spec.rb
+++ b/backend/spec/tasks/nico_sync_spec.rb
@@ -174,20 +174,20 @@ RSpec.describe 'nico:sync' do
 
   def create_post_version_for!(post, version_no: 1, event_type: 'create', created_by_user: nil)
     PostVersion.create!(
-      post: post,
-      version_no: version_no,
-      event_type: event_type,
-      title: post.title,
-      url: post.url,
-      thumbnail_base: post.thumbnail_base,
-      tags: snapshot_tags(post),
-      tags_json: post.snapshot_tags_json,
-      parent_post_ids: post.snapshot_parent_post_ids.join(' '),
-      original_created_from: post.original_created_from,
-      original_created_before: post.original_created_before,
-      created_at: Time.current,
-      created_by_user: created_by_user
-    )
+        event_id: create_event!(event_type: 'unknown').id,
+        post: post,
+        version_no: version_no,
+        event_type: event_type,
+        title: post.title,
+        url: post.url,
+        thumbnail_base: post.thumbnail_base,
+        tags: snapshot_tags(post),
+        tags_json: post.snapshot_tags_json,
+        parent_post_ids: post.snapshot_parent_post_ids.join(' '),
+        original_created_from: post.original_created_from,
+        original_created_before: post.original_created_before,
+        created_at: Time.current,
+        created_by_user: created_by_user)
   end
 
   it '新規 post 作成時に version 1 を作る' do
@@ -209,6 +209,7 @@ RSpec.describe 'nico:sync' do
     expect {
       run_rake_task('nico:sync')
     }.to change(PostVersion, :count).by(1)
+        .and change { event_rows.length }.by(1)
 
     post = Post.find_by!(url: 'https://www.nicovideo.jp/watch/sm9')
     version = post.post_versions.order(:version_no).last
@@ -217,6 +218,32 @@ RSpec.describe 'nico:sync' do
     expect(version.event_type).to eq('create')
     expect(version.created_by_user).to be_nil
     expect(version.tags).to eq(snapshot_tags(post.reload))
+    event = event_rows.last
+    expect(event).to include('event_type' => 'posts.sync', 'created_by_user_id' => nil)
+    expect(version.event_id).to eq(event.fetch('id'))
+  end
+
+  it 'groups created and updated post histories into one sync event' do
+    existing = create(:post, title: 'old', url: 'https://www.nicovideo.jp/watch/sm9')
+    stub_python([
+        { 'code' => 'sm9', 'title' => 'updated', 'tags' => [] },
+        { 'code' => 'sm10', 'title' => 'created', 'tags' => [] }])
+    allow(URI).to receive(:open).and_return(StringIO.new('<html></html>'))
+    previous_post_ids = PostVersion.pluck(:id)
+    previous_tag_ids = TagVersion.pluck(:id)
+
+    expect { run_rake_task('nico:sync') }.to change { event_rows.length }.by(1)
+
+    event = event_rows.last
+    expect(event).to include('event_type' => 'posts.sync', 'created_by_user_id' => nil)
+    created = Post.find_by!(url: 'https://www.nicovideo.jp/watch/sm10')
+    versions = PostVersion.where.not(id: previous_post_ids)
+    expect(versions.pluck(:post_id).uniq).to contain_exactly(existing.id, created.id)
+    expect(versions.pluck(:event_type)).to include('create', 'update')
+    expect(versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
+    tag_versions = TagVersion.where.not(id: previous_tag_ids)
+    expect(tag_versions).not_to be_empty
+    expect(tag_versions.pluck(:event_id).uniq).to eq([event.fetch('id')])
   end
 
   it '既存 post の内容または tags が変わったとき update version を作る' do
@@ -282,6 +309,7 @@ RSpec.describe 'nico:sync' do
     }])
 
     allow(URI).to receive(:open).and_return(StringIO.new('<html></html>'))
+    events = event_rows
 
     expect {
       run_rake_task('nico:sync')
@@ -291,6 +319,7 @@ RSpec.describe 'nico:sync' do
     expect(version.version_no).to eq(1)
     expect(version.event_type).to eq('create')
     expect(version.tags).to eq(snapshot_tags(post.reload))
+    expect(event_rows).to eq(events)
   end
 
   it '新規 nico tag に nico tag version を作る' do